1. Field of the Invention
This invention relates generally to the field of longwall mining and more particularly to a novel mine roof support structure and method of operation adapted for use in underground mining of thick mineral seams.
2. Description of the Prior Art
A known method of mining thick underground seams of coal or other mineral involves driving parallel head- and tailgate development entries into a mineral seam to define a panel to be mined and recovered in a retreating longwall operation. Typically, the seam is exploited by cutting a slice at the bottom and allowing the overlying mineral to cave.
The mechanization of thick seam longwalling with induced sublevel caving in the above manner is reported, for example, in the Mining Congress Journal for Dec., 1972, in an article entitled "Longwall Mining with Sublevel Caving". The longwall face extending between head- and tailgate entries may be cut bi-directionally with a double-drum shearer which loads an armored chain-type face conveyor at the solid face under the protection of a row of self-advancing adjacent mine roof supports, each extending transversely to the solid face. As the shearer passes each position along the face, the roof supports are progressively advanced to support the newly exposed mine roof while mineral from overlying strata on the gob side is allowed to cave for subsequent loading by plow or planer on a gob side chain conveyor extending along the opposite side of the row of roof supports.
Most of the production comes from the caved mineral, which means that recovery is importantly affected by how quickly and completely the overlying mineral caves and how the mineral-gob interface behaves. Factors to be considered in this connection are the friability of the mineral, the height of the seam, and the type of rock overlying the initially caved mineral. The manner in which caving is induced and the technique for loading the caved mineral, in synchronization with loading operations on the face side, particularly where the distance between face and gob conveyors varies, are considerations of primary importance in the present invention.
The essential intermediate element in a mechanized system of this sort is the roof support. Existing roof support structures are provided with hydraulic rams by means of which the face and the gob sides of each transversely extending support structure may be attached respectively to the face and gob chain conveyors. In operation, one mining team can be occupied with the shearing and the movement of the face conveyor while another team may be working on the drawing of caved mineral, the advancement of the supports, and the advancement of the gob conveyor. In the European system the roof support is typically supplied by means of so-called "walking" supports. These may, for example, be so-called two-step or double action assemblies wherein a pair of spaced apart parallel monolithic roof supporting frames or roof bars, each carried above a separate base by a number of pressurizable hydraulic props, are interconnected by advancing means positioned between them. By alternately anchoring and depressurizing the respective props of the two frames, the units may be successively advanced toward the solid mineral face, each using the other in turn as an abutment.
With supports as described, advancement of the face and gob conveyors may be performed by operating the rams at the two extremities of each support structure simultaneously with the advance of the supports themselves. However, even though the walking type support is articulated, its two sections are moved as a part of a single advancing operation. In other words, mining does not proceed with one unit or section advanced with respect to the other. This in turn interrupts the continuity of mining operations on the face and gob sides and hence the rate of mineral recovery. The essentially monolithic character of the roof structures also lessens their ability to accommodate varying distances between face and gob conveyors.
A related problem is longwalling which must be dealt with is how to provide prompt support for the roof area exposed by the lateral advance of the shearing machine along the longwall face. One way to do this is to incorporate hydraulically operated telescoping extension in the roof contacting superstructures of the supports. These extensions may be advanced to the solid face behind the advancing shearer without interruption of the mining cycle and without moving the upstanding hydraulic props. However, the cantilevered length of load bearing frame or bar is thereby increased, and the support density is thereby weakened. Since there is normally a considerable separation between the adjacent frames at interconnected roof support units, wire mesh or netting must be placed against the exposed roof between them to further contain fractured mineral. Such wire netting is also used to protect the gob conveyor but inevitably slows down the recovery of caved mineral which must be tapped through windows cut in the netting. In order to accelerate the flow of caved mineral through windows in the wire netting, curved, hinged aft roof support sections called "bananas" may be "pumped" or flexed. It has also been suggested that these banana canopies may be replaced by articulated shields which can be retracted during sublevel caving.